If the area is covered by a mile of ice, then how do they know that these
hi-alt sediments are fossiliferous?

I'm sure nobody knows if there are any sediments there. The mountain range,
which is probably only known from radar, came up (oops... sorry for the pun)
as the answer to the question of where ice might have survived since the
Cretaceous (probably in the context of surviving DNA -- I forgot).